In electrophotography, a latent image is created on the surface of an imaging member such as a photoconducting material by first uniformly charging the surface and then selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between those areas on the surface which are exposed to light and those areas on the surface which are not exposed to light. The latent electrostatic image is developed into a visible image by electrostatic toners. The toners are selectively attracted to either the exposed or unexposed portions of the photoconductor surface, depending on the relative electrostatic charges on the photoconductor surface, the development electrode and the toner.
Typically, a dual layer electrophotographic photoconductor comprises a substrate such as a metal ground plane member on which a charge generation layer (CGL) and a charge transport layer (CTL) are coated. The charge transport layer contains a charge transport material which comprises a hole transport material or an electron transport material. For simplicity, the following discussions herein are directed to use of a charge transport layer which comprises a hole transport material as the charge transport compound. One skilled in the art will appreciate that if the charge transport layer contains an electron transport material rather than a hole transport material, the charge placed on a photoconductor surface will be opposite that described herein.
Generally, when the charge transport layer containing a hole transport material is formed on the charge generation layer, a negative charge is typically placed on the photoconductor surface. Conversely, when the charge generation layer is formed on the charge transport layer, a positive charge is typically placed on the photoconductor surface. Conventionally, the charge generation layer comprises a polymeric binder containing a charge generating compound or molecule while the charge transport layer comprises a polymeric binder containing the charge transport compound or molecule. The charge generating compounds within the CGL are sensitive to image-forming radiation and photogenerate electron-hole pairs within the CGL as a result of such radiation. The CTL is usually non-absorbent of the image-forming radiation and the charge transport compounds serve to transport holes to the surface of a negatively charged photoconductor. Photoconductors of this type are disclosed in the Adley et al U.S. Pat. No. 5,130,215 and the Balthis et al U.S. Pat. No. 5,545,499.
Generally, various materials which are included in the photoconductor are sensitive to oxidative degradation. Typically, antioxidants are incorporated into the charge transport layer in order to improve the resistance of the photoconductor to oxidative degradation. For example, the Iwasaki et al U.S. Pat. No. 5,192,633, the Shigematsu et al U.S. Pat. No. 5,595,846 and the Kishi et al U.S. Pat. No. 5,670,284 all disclose the use of antioxidants in a charge transfer layer of a dual layer photoconductor in order to improve the oxidative degradation resistance of the photoconductor.
Unfortunately, many commonly employed antioxidants have been determined to significantly increase photoconductor fatigue, both initially and upon cycling, when incorporated into the charge transport layer. Generally, increased fatigue is evidenced by increases in the discharge voltage of the photoconductor, either initially or upon cycling. Fatigue is undesirable as it can reduce the development vector thereby resulting in light or washed out print as well as print that varies from page to page. Accordingly, there is a continuing need for improved photoconductors which exhibit good resistance to oxidative degradation while maintaining good sensitivity, stability and durability.